Why is the "dislike" button going into extinction?

I've noticed that Facebook and now Google+ only have a "Like" (or +1) button, in spite of users wanting a "Dislike" button. This seems to be a bit of trend. Disqus also dropped their dislike vote - now you can only "like" stuff.

Is there any particular reason for that (other than may be having one less button and less functionality to worry about)? This always struck me as a bad idea because it's harder to identify bad content (eg: stuff that would get massively downvoted but can garner some upvotes).